Among refractory products, fused and cast products are well known for the fabrication of glass fusion furnaces and for sintered products.
As opposed to sintered products, the fused and cast products more often undergo an intergranular vitreous phase binding the crystallized grains. The problems posed by sintered products as well as fused and cast products and the technical solutions adopted to resolve such problems are generally different from one another. A composition developed to manufacture a sintered product is therefore, a priori, not readily usable to manufacture a fused and cast product and vice versa.
Fused and cast products, often referred to as electrocasts, are obtained by melting a mixture of appropriate raw materials in an electric arc furnace or by any other technique suited to these types of products. The molten material is then poured into a mold and the resulting product is subsequently subjected to a controlled cooling cycle with the objective of cooling the product down to room temperature without causing any fractures. This process is referred as “annealing” by professionals skilled in the art.
Among fused and cast products, electrocast products having a high concentration of zirconia, that is to say, comprising more than 85% zirconia (ZrO2), are well known for their ability to resist corrosion without any coloration of the glass and without producing any defects.
Conventionally, fused and cast products having a high concentration of zirconia also include sodium oxide (Na2O) in order to avoid the formation of zircon from the zirconia and silica present in the product. The formation of zircon is detrimental as it is accompanied by a reduction in volume of up to 20%, which creates certain mechanical constraints responsible for cracks.
Product ER-1195 produced and sold by Société Européenne des Produits Réfractaires and covered under patent EP-B-403 387 is currently widely used in glass furnaces. The chemical composition of this product is roughly 94% zirconia, 4 or 5% silica, approximately 1% alumina, 0.3% sodium oxide and less then 0.05% P2O5 by weight. This composition is typical of high concentration zirconia products used in glass furnaces.
FR 2 701 022 discloses fused and cast products having a high zirconia concentration which contain 7.0 to 11.2% SiO2 by weight, 0.05 to 1% P2O5 by weight, 0.05 to 1.0% boric oxide B2O3 by weight and 0.01 à 0.12% of Na2O+K2O by weight.
FR 2 723 583 describes fused and cast products having a high zirconia concentration which contain 3 to 8% SiO2 by weight, 0.1 to 2% Al2O3 by weight, 0.05 to 3% boric oxide B2O3 by weight, 0.05 to 3% of BaO+SrO+MgO by weight and 0.05 to 0.6% Na2O+K2O by weight, and less than 0.3% of Fe2O3+TiO2 by weight.
The creep of a material subjected to pressure (compression, tensile force or bending) may be defined as the capacity of the material to deform itself visco-plastically, that is to say, in a permanent manner under the influence of this load.
Current high quality glass development has increased the requisites concerning refractory products for glass fusion furnaces. In particular, there is a need for refractory products that present improved creep properties and/or resistance to thermal variations, or, more generally, products that present a better compromise between these two properties.
The object of the present invention is to satisfy this need.